The present disclosure relates to systems and methods for magnetic resonance (“MW”) imaging. More particularly, the invention relates to systems and methods for magnetization transfer (“MT”) using MR techniques.
Any nucleus that possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency), which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant γ of the nucleus). Nuclei which exhibit these phenomena are referred to herein as “spins.”
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment MZ is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a transient electromagnetic pulse (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides on signals that are emitted by the excited spins after the pulsed excitation signal B1 is terminated. Depending upon chemically and biologically determined variable parameters such as proton density, longitudinal relaxation time (“T1”) describing the recovery of MZ along the polarizing field, and transverse relaxation time (“T2”) describing the decay of Mt in the x-y plane, this nuclear magnetic resonance (“NMR”) phenomena is exploited to obtain image contrast and concentrations.
Magnetic resonance imaging (“MRI”) can provide unique information about the chemical and molecular environment of imaged samples or tissues compared to other modalities. One type of measurement is broadly termed magnetization transfer (“MT”), and involves measuring signals from nuclei of hydrogen atoms whose motion is restricted because they are bound to macromolecules. These so-called “bound pool” spins or spin species have very short transverse relaxation times T2 due to tight coupling to their environment, and are hence difficult to image. This is because in traditional MRI methods, T2 needs to be sufficiently long (i,e, greater than about 10 ms) to allow for spatial encoding gradients between excitation and acquisition to be played out before signals are completely decayed. For some bound pool spin species, however, T2 can be as short as 10-50 μs.
Even though bound pool spin signals decay rapidly due to short T2, they can exchange magnetization with so-called “free pool” spins, which are loosely bound to their environment, and typically present as water protons in many soft tissue environments. The magnetization exchange, occurring via various spin exchange processes, allows the properties of the bound pool to be probed by the application of RF excitations. In particular, applied excitations at frequencies offset from water would have little direct effect on the free pool, but will saturate the bound pool, and generate measurable signals. This is because, in general, hound pool spins have a much broader absorption line shapes compared to free pool spins, making them more sensitive to appropriate off-resonance irradiation. As a result, characteristics of macromolecular components of tissue that would otherwise be unobservable because of their very short T2 can be measurable.
In traditional MT measurements, RF saturation power is applied at one off-resonance frequency and signal effects are observed, typically in the steady state. In some alternatives, different RF power and off-resonance frequencies may be used, and fit to quantitative models with potentially more specificity. However, such models assume a single transferring compartment with a single line shape, and can produce similar results for very different tissues.
Therefore, there is a need for improved magnetization transfer MRI techniques that provide high sensitivity to tissue differences and can overcome constraints of the limited safe power range.